1. Field of the Invention
This invention relates to the field of devices that protect refrigeration compressors from damage due to liquid slugging.
2. Description of Prior Art
For typical refrigeration systems, lubricating oil and liquid refrigerant can sporadically enter the suction piping connected to the compressors. This event is commonly called slugging, or sometimes called flood-back. Refrigeration compressors are specifically designed to pump only vapor and can be catastrophically damaged if they are forced to pump liquids. Therefore, methods have been developed to capture any liquid in the suction piping and thus reduce the likelihood that compressor damage will occur due to slugging.
The oil that can form slugs in the suction piping originates in the refrigeration compressors. This oil is normally held within the compressor crankcase and is thus constantly available to lubricate the compressor bearings and sliding parts.
During normal operation, some oil leaves the compressor with the discharge vapor as a mist and enters the refrigeration piping system. Methods and devices have been designed to collect most this oil leaving a compressor and properly returning this oil to the compressor crankcase. The most common method incorporates a device called an oil separator. Although many types of oil separators have been develop, they all strive to trap the oil mist and coalesce it back into a liquid. The captured oil is then directed to a reservoir, where it is stored until required to replenish the compressor crankcases.
A specialized device, called an oil float valve, controls the flow of oil from the oil reservoir to the compressor crankcase. The float valve senses the oil level within the crankcase. When a low oil level is sensed, the float valve allows oil to flow from the reservoir to the crankcase. Conversely, when a high oil level is sensed, the float valve prevents oil from flowing from the reservoir to the crankcase.
Oil separators and oil float valves have been proven highly reliable and therefore are widely used by the refrigeration industry. As an example of a modern oil float valve design, U.S. Pat. No. 5,901,559 discloses an electromechanical concept that is claimed to provide very stable performance.
The most common cause of liquid refrigerant in the suction piping is unstable expansion valve operation. The purpose of the expansion valve is to maintain the suction gas in a slightly superheated state, thereby striving to keep the suction piping free of liquid refrigerant. Even when properly sized, expansion valves sometimes become unstable and allow liquid refrigerant to enter the suction piping.
Since oil separators are not 100% efficient and expansion valves are not 100% reliable, the potential for liquid slugging exists even for well-designed refrigeration systems. A commonly used method of reducing the likelihood of compressor damage due to this inadvertent liquid slugging is to install a device in the suction piping that will trap the liquid before it reaches the compressors. This device is commonly called a suction trap, surge drum, knockout drum, or suction accumulator. The suction trap is a vessel that is substantially larger in volume than the suction piping. The velocity of the suction gas is thus reduced when it enters this large vessel, thereby promoting the entrained liquids to separate from the gas and settle to the bottom of the vessel.
The liquid that settles to the bottom of the suction trap can be either liquid refrigerant or oil, both of which must be reintroduce into the refrigeration system. Typically, these liquids are reintroduced back into the refrigeration system by slowly allowing these liquids to flow, or "bleed", into the suction piping located downstream of the suction trap. This "bleed" rate is usually controlled by a small valve or orifice. The valve or orifice is sized to produce a flow that hopefully is slow enough not to cause damage to the compressors.
ASHRAE Handbook, Refrigeration-1998, Chapter 2: "System Practices for Halocarbon Refrigerants", FIG. 17 and FIG. 34 provide some guidelines for designing suction traps. The metered liquid from the suction trap is heated, either with an electric heater or with a heat exchanger using the warm liquid refrigerant from the condenser. In this manner, refrigerant in the metered stream has a chance to be boil from a liquid to a gas before it is reintroduced into the refrigeration system. To further guard against the presence of refrigerant within the metered liquid, U.S. Pat. No. 4,068,493 describes the implementation of a thermostatic expansion valve, with its sensing bulb attached to the pipe carrying the metered liquid. In this manner, the thermostatic expansion valve stops the metered flow if it detects the presence of refrigerant, as indicated by a low superheat measurement.
ASHRAE Handbook, Refrigeration-1998, Chapter 2: "System Practices for Halocarbon Refrigerants", FIG. 17 also teaches that to achieve additional protection against compressor damage, the liquid from the suction trap can be stored in a receiver, commonly called a reservoir. The stored liquid is heated with an electric heater and then reintroduced into the compressor crankcases through float valves connected to each compressor crankcase. Since the reservoir and crankcase are essentially at the same pressure, the reservoir must be elevated above the crankcases to allow the oil to drain from the reservoir. Since it is sometimes inconvenient to substantially elevate the reservoir above the crankcases, U.S. Pat. No. 4,530,215 describes the use of a mechanical pump to force the oil from the reservoir to the crankcases.
ASHRAE Handbook, Refrigeration-1998, Chapter 2: "System Practices for Halocarbon Refrigerants", section titled PIPING AT MULTIPLE COMPRESSOR: Suction Piping also suggests that a suction trap can be constructed from the pipe that interconnects multiple compressors. This interconnecting piping is called the suction header. The ASHRAE Handbook states, "the suction header may be designed to function as a suction trap. The suction header should be large enough to provide a region of low velocity within the header to allow the suction gas and oil to separate". As an example of this concept, U.S. Pat. No. 4,554,795 discloses the use of the suction header as a suction trap, with the implementation of oil pick-up devices on each compressor suction line, which promote the oil to flow to the compressors that are running.
Another method of dealing with the liquid inside the suction trap for ammonia systems is described in ASHRAE Handbook, Refrigeration-1998, Chapter 3: "System Practices for Ammonia Refrigerants, FIG. 6 and FIG. 7. These illustrations show the transferring of the liquid refrigerant and oil from the low-pressure suction trap to the high-pressure liquid refrigerant receiver. This method uses a vessel called a transfer drum that can be alternately vented to either the low-pressure or the high pressure via solenoid valves. First, the transfer drum is vented to the low-pressure and allowed to fill up with the liquid from the suction trap. When the transfer drum is filled, a float switch is activated and then the vent is switched to the high pressure. Then the liquid refrigerant is drained from the transfer drum to the liquid refrigerant receiver, either with a pump or via gravity. After the transfer drum is empty, the float switch is deactivated and the vent is switched back to low-pressure and the cycle is repeated. Since oil and ammonia are nearly immiscible, the oil that is transferred from the suction trap to the receiver settles to the bottom of the receiver and is periodically drained. This method is not suitable for halocarbon systems because oil and halocarbon refrigerants are miscible. Therefore, there is no means to extract the oil from the refrigerant and then return it to the compressors.
In summary, the conventional methods for abating compressor damage due to liquid slugging are well documented and widely used today. Nevertheless, these methods suffer from several disadvantages:
(a) The method of metering the liquid from the suction trap back into the suction pipe is only effective for average slugging situations. But sometimes, system malfunctions can cause a large amount of liquid refrigerant or oil to travel through the suction pipe, resulting in a steady and persistent bleed flow. Under this situation, the conventional methods of heating the bleed flow to boil the liquid refrigerant into a vapor may be insufficient. A preferred method would be to use a device that would ensure sufficient heat to boil the entire liquid refrigerant in the bleed flow, for even the most severe flood-back conditions. Also preferred would be a heating method that does not require expensive electric heat or require the use of a heat exchanger. PA1 (b) The method of feeding oil through float valves from a low-pressure storage reservoir to the compressor crankcases can be cumbersome to install because the reservoir must be substantially elevated above the compressors to cause the oil to properly flow. A preferred method would be to maintain the oil storage reservoir at a pressure higher than the crankcase pressure. In this manner, a reliable oil flow through the float valves will be assured, even if the receiver is not elevated above the compressors. PA1 (c) The method of transferring the liquid from the suction trap to the liquid refrigerant receiver is not suitable for halocarbon systems because the oil and liquid refrigerant are miscible and therefore the oil cannot be extracted from the refrigerant and properly returned to the compressor crankcases. It would be preferred to implement a method where the oil can be readily extracted from the refrigerant and then reliably returned to the compressor crankcases.
What is needed, therefore, is a device to prevent compressor damage due to liquid slugging that can handle the most severe flood-back situations. What is further needed is a device to prevent compressor damage due to liquid slugging that does not require an electric heater or a heat exchanger to boil off the liquid refrigerant within the slug. What is yet further needed is device to prevent compressor damage due to liquid slugging that can return the oil within the slug back to the compressor crankcases in a highly reliable and easily implemented manner.